U.S. patent application number 15/465502 was filed with the patent office on 2018-09-27 for concentration management in flow battery systems using an electrochemical balancing cell.
The applicant listed for this patent is LOCKHEED MARTIN ADVANCED ENERGY STORAGE, LLC. Invention is credited to Kean DUFFEY, Sophia LEE, Jeremy LORETZ.
Application Number | 20180277868 15/465502 |
Document ID | / |
Family ID | 63582963 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180277868 |
Kind Code |
A1 |
LORETZ; Jeremy ; et
al. |
September 27, 2018 |
CONCENTRATION MANAGEMENT IN FLOW BATTERY SYSTEMS USING AN
ELECTROCHEMICAL BALANCING CELL
Abstract
During operation of flow battery systems, the volume of one or
more electrolyte solutions can change due to solvent loss
processes. An electrochemical balancing cell can be used to combat
volume variability. Methods for altering the volume of one or more
electrolyte solutions can include: providing a first
electrochemical balancing cell containing a membrane disposed
between two half-cells, establishing fluid communication between a
first aqueous electrolyte solution of a flow battery system and a
first half-cell of the first electrochemical balancing cell, and
applying a current to the first electrochemical balancing cell to
change a concentration of one or more components in the first
aqueous electrolyte solution. Applying the current causes water to
migrate across the membrane, either to or from the first aqueous
electrolyte solution, and a rate of water migration is a function
of current.
Inventors: |
LORETZ; Jeremy; (Boston,
MA) ; DUFFEY; Kean; (Brighton, MA) ; LEE;
Sophia; (Philadelphia, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LOCKHEED MARTIN ADVANCED ENERGY STORAGE, LLC |
Bethesda |
MD |
US |
|
|
Family ID: |
63582963 |
Appl. No.: |
15/465502 |
Filed: |
March 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/188 20130101;
Y02B 90/10 20130101; Y02E 60/50 20130101 |
International
Class: |
H01M 8/04276 20060101
H01M008/04276; H01M 8/18 20060101 H01M008/18 |
Claims
1. A method comprising: providing a first electrochemical balancing
cell comprising a membrane disposed between two half-cells;
establishing fluid communication between a first aqueous
electrolyte solution of a flow battery system and a first half-cell
of the first electrochemical balancing cell; and applying a current
to the first electrochemical balancing cell to change a
concentration of one or more components in the first aqueous
electrolyte solution; wherein applying the current causes water to
migrate across the membrane, either to or from the first aqueous
electrolyte solution, and a rate of water migration is a function
of current.
2. The method of claim 1, wherein a second half-cell of the first
electrochemical balancing cell contains a balancing aqueous fluid,
and applying the current causes water to migrate from the second
half-cell into the first aqueous electrolyte solution in the first
half-cell, thereby decreasing an active material concentration in
the first aqueous electrolyte solution.
3. The method of claim 2, wherein the second half-cell of the first
electrochemical balancing cell contains an oxygen-generation
catalyst, and applying the current also generates protons from the
balancing aqueous fluid; wherein the protons also migrate from the
second half-cell into the first aqueous electrolyte solution in the
first half-cell.
4. The method of claim 2, further comprising: removing the
balancing aqueous fluid from the second half-cell; and after
removing the balancing aqueous fluid, applying the current to the
first electrochemical balancing cell while the second half-cell is
empty; wherein applying the current to the first electrochemical
balancing cell while the second half-cell is empty causes water to
migrate from the first half-cell into the second half-cell, thereby
increasing an active material concentration in the first aqueous
electrolyte solution.
5. The method of claim 1, wherein a second half-cell of the first
electrochemical balancing cell is left empty, and applying the
current to the first electrochemical balancing cell causes water to
migrate from the first half-cell into the second half-cell, thereby
increasing an active material concentration in the first aqueous
electrolyte solution.
6. The method of claim 5, further comprising: introducing a
balancing aqueous fluid into the second half-cell; and after
introducing the balancing aqueous fluid into the second half-cell,
applying the current to the first electrochemical balancing cell;
wherein applying the current to the first electrochemical balancing
cell causes water to migrate from the second half-cell into the
first half-cell, thereby decreasing an active material
concentration in the first aqueous electrolyte solution.
7. The method of claim 6, wherein the second half-cell of the first
electrochemical balancing cell contains an oxygen-generation
catalyst, and applying the current also generates protons from the
balancing aqueous fluid; wherein the protons also migrate from the
second half-cell into the first aqueous electrolyte solution in the
first half-cell.
8. The method of claim 1, wherein a state of charge of the first
aqueous electrolyte solution also changes while applying the
current to the first electrochemical balancing cell.
9. The method of claim 1, wherein the first aqueous electrolyte
solution is circulated through a negative half-cell of the flow
battery system.
10. The method of claim 1, wherein a second aqueous electrolyte
solution of the flow battery system is in fluid communication with
a second electrochemical balancing cell.
11. A method comprising: providing a first electrochemical
balancing cell comprising a membrane disposed between two
half-cells; establishing fluid communication between a first
aqueous electrolyte solution of a flow battery system and a first
half-cell of the first electrochemical balancing cell; determining
a quantity of the first aqueous electrolyte solution in the flow
battery system; applying a current to the first electrochemical
balancing cell; and either introducing a balancing aqueous fluid to
a second half-cell of the first electrochemical balancing cell or
emptying the second half-cell of the first electrochemical
balancing cell in response to the quantity of the first aqueous
electrolyte solution that is determined; wherein applying the
current causes water to migrate across the membrane into the first
aqueous electrolyte solution when the balancing aqueous fluid is
present in the second half-cell of the first electrochemical
balancing cell, and applying the current causes water to migrate
across the membrane into the second half-cell when the balancing
aqueous fluid is absent from the second half-cell of the first
electrochemical balancing cell, and a rate of water migration is a
function of current.
12. The method of claim 11, wherein the second half-cell of the
first electrochemical balancing cell contains an oxygen-generation
catalyst, and applying the current to the first electrochemical
balancing cell also generates protons when the balancing aqueous
fluid is present; wherein the protons also migrate from the second
half-cell into the first aqueous electrolyte solution in the first
half-cell.
13. The method of claim 11, wherein the balancing aqueous fluid is
water or an aqueous electrolyte solution.
14. The method of claim 11, wherein a state of charge of the first
aqueous electrolyte solution also changes while applying the
current to the first electrochemical balancing cell.
15. The method of claim 11, wherein the first aqueous electrolyte
solution is circulated through a negative half-cell of the flow
battery system.
16. The method of claim 11, wherein a second aqueous electrolyte
solution of the flow battery system is in fluid communication with
a second electrochemical balancing cell.
17. A flow battery system comprising: a first half-cell containing
a first aqueous electrolyte solution; a second half-cell containing
a second aqueous electrolyte solution; a first electrochemical
balancing cell comprising a membrane disposed between two
half-cells; wherein either the first half-cell or the second
half-cell of the flow battery system is in fluid communication with
a first half-cell of the first electrochemical balancing cell; and
a source of a balancing aqueous fluid in fluid communication with
the first electrochemical balancing cell, the flow battery system
being configured to introduce the balancing aqueous fluid to a
second half-cell of the first electrochemical balancing cell when a
quantity of the first aqueous electrolyte solution falls below a
lower threshold and to withdraw the balancing aqueous fluid from
the second half-cell of the first electrochemical balancing cell
when the quantity of the first aqueous electrolyte solution exceeds
an upper threshold.
18. The flow battery system of claim 17, further comprising: a
detector configured to determine the quantity of the first aqueous
electrolyte solution.
19. The flow battery system of claim 17, further comprising: a
processor responsive to the quantity of the first aqueous
electrolyte solution and configured to initiate introduction or
withdrawal of the first aqueous electrolyte solution to or from the
second half-cell of the first electrochemical balancing cell.
20. The flow battery system of claim 17, wherein the second
half-cell of the first electrochemical balancing cell contains an
oxygen-generation catalyst.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD
[0003] The present disclosure generally relates to energy storage
and, more specifically, to techniques for managing the
concentrations of components within one or more electrolyte
solutions used in flow battery systems.
BACKGROUND
[0004] Electrochemical energy storage systems, such as batteries,
supercapacitors and the like, have been widely proposed for
large-scale energy storage applications. Various battery designs,
including flow batteries, have been considered for this purpose.
Compared to other types of electrochemical energy storage systems,
flow batteries can be advantageous, particularly for large-scale
applications, due to their ability to decouple the parameters of
power density and energy density from one another.
[0005] Flow batteries generally include negative and positive
active materials in corresponding electrolyte solutions, which are
flowed separately across opposing faces of a membrane or separator
in an electrochemical cell containing negative and positive
electrodes. The flow battery is charged or discharged through
electrochemical reactions of the active materials that occur inside
the two half-cells. As used herein, the terms "active material,"
"electroactive material," "redox-active material" or variants
thereof synonymously refer to a material that undergoes a change in
oxidation state during operation of a flow battery or like
electrochemical energy storage system (i.e., during charging or
discharging).
[0006] Although flow batteries hold significant promise for
large-scale energy storage applications, they have often been
plagued by sub-optimal energy storage performance (e.g., round trip
energy efficiency) and limited cycle life, among other factors.
Despite significant investigational efforts, no commercially viable
flow battery technologies have yet been developed. Certain issues
leading to poor energy storage performance, limited cycle life, and
other performance-degrading factors are discussed hereinafter.
[0007] One issue occurring commonly during operation of flow
batteries is that the active material concentration in one or more
of the electrolyte solutions can change over time. Although
degradation of an active material could result in a concentration
decrease, the more common manner in which the active material
concentration can change is through gain or loss of solvent. In an
aqueous electrolyte solution, for example, loss of water can lead
to an increase in the concentration of the active material. Water
loss from an aqueous electrolyte solution can occur through a
variety of means during operation of a flow battery such as, for
example, due to heating during electrochemical reactions and/or
when venting to release a gas generated during parasitic reactions,
which are described further herein. In some cases, an aqueous
electrolyte solution can gain water, thereby decreasing the active
material concentration.
[0008] In view of the foregoing, flow battery systems capable of
managing the concentrations of various components in an electrolyte
solution would be highly desirable in the art. The present
disclosure satisfies the foregoing needs and provides related
advantages as well.
SUMMARY
[0009] In some embodiments, methods for transporting water to or
from an aqueous electrolyte solution in a flow battery are
described herein. The methods include: providing a first
electrochemical balancing cell containing a membrane disposed
between two half-cells; establishing fluid communication between a
first aqueous electrolyte solution of a flow battery system and a
first half-cell of the first electrochemical balancing cell; and
applying a current to the first electrochemical balancing cell to
change a concentration of one or more components in the first
aqueous electrolyte solution. Applying the current causes water to
migrate across the membrane, either to or from the first aqueous
electrolyte solution. A rate of water migration is a function of
current.
[0010] In other various embodiments, methods for transporting water
to or from an aqueous electrolyte solution in a flow battery can
include: providing a first electrochemical balancing cell
containing a membrane disposed between two half-cells; establishing
fluid communication between a first aqueous electrolyte solution of
a flow battery system and a first half-cell of the first
electrochemical balancing cell; determining a quantity of the first
aqueous electrolyte solution in the flow battery system; applying a
current to the first electrochemical balancing cell; and either
introducing a balancing aqueous fluid to a second half-cell of the
first electrochemical balancing cell or emptying the second
half-cell of the first electrochemical balancing cell in response
to the quantity of the first aqueous electrolyte solution that is
determined. Applying the current causes water to migrate across the
membrane into the first aqueous electrolyte solution when the
balancing aqueous fluid is present in the second half-cell of the
first electrochemical balancing cell. Applying the current causes
water to migrate across the membrane into the second half-cell when
the balancing aqueous fluid is absent from the second half-cell of
the first electrochemical balancing cell. A rate of water migration
is a function of current.
[0011] In still other various embodiments, the present disclosure
describes flow battery systems including: a first half-cell
containing a first aqueous electrolyte solution; a second half-cell
containing a second aqueous electrolyte solution; a first
electrochemical balancing cell containing a membrane disposed
between two half-cells; and a source of a balancing aqueous fluid
in fluid communication with the electrochemical balancing cell.
Either the first half-cell or the second half-cell of the flow
battery system is in fluid communication with a first half-cell of
the first electrochemical balancing cell. The flow battery system
is configured to introduce the balancing aqueous fluid to a second
half-cell of the first electrochemical balancing cell when a
quantity of the first aqueous electrolyte solution falls below a
lower threshold and to withdraw the balancing aqueous fluid from
the second half-cell of the first electrochemical balancing cell
when the quantity of the first aqueous electrolyte solution exceeds
an upper threshold.
[0012] The foregoing has outlined rather broadly the features of
the present disclosure in order that the detailed description that
follows can be better understood. Additional features and
advantages of the disclosure will be described hereinafter. These
and other advantages and features will become more apparent from
the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the present disclosure,
and the advantages thereof, reference is now made to the following
descriptions to be taken in conjunction with the accompanying
drawings describing specific embodiments of the disclosure,
wherein:
[0014] FIG. 1 depicts a schematic of an illustrative flow battery
containing a single electrochemical cell; and
[0015] FIG. 2 shows the illustrative flow battery system of FIG. 1
further incorporating electrochemical balancing cells in fluid
communication with each electrolyte solution.
DETAILED DESCRIPTION
[0016] The present disclosure is directed, in part, to flow
batteries containing an electrochemical balancing cell in fluid
communication with at least one electrolyte solution. The present
disclosure is also directed, in part, to methods for managing the
active material concentration in at least one electrolyte solution
using one or more electrochemical balancing cells.
[0017] The present disclosure may be understood more readily by
reference to the following description taken in connection with the
accompanying figures and examples, all of which form a part of this
disclosure. It is to be understood that this disclosure is not
limited to the specific products, methods, conditions or parameters
described and/or shown herein. Further, the terminology used herein
is for purposes of describing particular embodiments by way of
example only and is not intended to be limiting unless otherwise
specified. Similarly, unless specifically stated otherwise, any
description herein directed to a composition is intended to refer
to both solid and liquid versions of the composition, including
solutions and electrolytes containing the composition, and
electrochemical cells, flow batteries, and other energy storage
systems containing such solutions and electrolytes. Further, it is
to be recognized that where the disclosure herein describes an
electrochemical cell, flow battery, or other energy storage system,
it is to be appreciated that methods for operating the
electrochemical cell, flow battery, or other energy storage system
are also implicitly described.
[0018] It is also to be appreciated that certain features of the
present disclosure may be described herein in the context of
separate embodiments for clarity purposes, but may also be provided
in combination with one another in a single embodiment. That is,
unless obviously incompatible or specifically excluded, each
individual embodiment is deemed to be combinable with any other
embodiment(s) and the combination is considered to represent
another distinct embodiment. Conversely, various features of the
present disclosure that are described in the context of a single
embodiment for brevity's sake may also be provided separately or in
any sub-combination. Finally, while a particular embodiment may be
described as part of a series of steps or part of a more general
structure, each step or sub-structure may also be considered an
independent embodiment in itself.
[0019] Unless stated otherwise, it is to be understood that each
individual element in a list and every combination of individual
elements in that list is to be interpreted as a distinct
embodiment. For example, a list of embodiments presented as "A, B,
or C" is to be interpreted as including the embodiments "A," "B,"
"C," "A or B," "A or C," "B or C," or "A, B, or C."
[0020] In the present disclosure, the singular forms of the
articles "a," "an," and "the" also include the corresponding plural
references, and reference to a particular numerical value includes
at least that particular value, unless the context clearly
indicates otherwise. Thus, for example, reference to "a material"
is a reference to at least one of such materials and equivalents
thereof.
[0021] In general, use of the term "about" indicates approximations
that can vary depending on the desired properties sought to be
obtained by the disclosed subject matter and is to be interpreted
in a context-dependent manner based on functionality. Accordingly,
one having ordinary skill in the art will be able to interpret a
degree of variance on a case-by-case basis. In some instances, the
number of significant figures used when expressing a particular
value may be a representative technique of determining the variance
permitted by the term "about." In other cases, the gradations in a
series of values may be used to determine the range of variance
permitted by the term "about." Further, all ranges in the present
disclosure are inclusive and combinable, and references to values
stated in ranges include every value within that range.
[0022] As discussed above, energy storage systems that are operable
on a large scale while maintaining high efficiency values can be
extremely desirable. Flow batteries have generated significant
interest in this regard, but there remains considerable room for
improving their operating characteristics. One issue that can
complicate the operation of flow batteries is the alteration of
active material concentrations and other component concentrations
over the operational lifetime of a flow battery. Active material
concentrations deviating in either direction (i.e., high or low)
from the optimal working concentration range can be detrimental. An
overly high active material concentration, for example, can exceed
the solubility limit and result in damaging precipitation within
the flow battery components. Low active material concentrations, in
contrast, can lead to poor energy density values. Other components
of an electrolyte solution that are out of a desired concentration
range can be similarly problematic.
[0023] Before discussing further specifics of the flow battery
systems and methods of the present disclosure, illustrative flow
battery configurations and their operating characteristics will
first be described in greater detail hereinafter.
[0024] FIG. 1 depicts a schematic of an illustrative flow battery
containing a single electrochemical cell. Although FIGURE, 1 shows
a flow battery containing a single electrochemical cell, approaches
for combining multiple electrochemical cells together in an
electrochemical cell stack are known and are discussed hereinbelow.
Unlike typical battery technologies (e.g., Li-ion, Ni-metal
hydride, lead-acid, and the like), where active materials and other
components are housed in a single assembly, flow batteries
transport (e.g., via pumping) redox-active energy storage materials
from storage tanks through an electrochemical cell stack. This
design feature decouples the electrical energy storage system power
from the energy storage capacity, thereby allowing for considerable
design flexibility and cost optimization to be realized.
[0025] As shown in FIG. 1, flow battery 1 includes an
electrochemical cell that features separator 20 (e.g., a membrane)
that separates the two electrodes 10 and 10' of the electrochemical
cell. As used herein, the terms "separator" and "membrane"
synonymously refer to an ionically conductive and electrically
insulating material disposed between the positive and negative
electrodes of an electrochemical cell. Electrodes 10 and 10' are
formed from a suitably conductive material, such as a metal,
carbon, graphite, and the like. Although FIG. 1 has shown
electrodes 10 and 10' as being spaced apart from separator 20,
electrodes 10 and 10' can also be abutted with separator 20 in more
particular embodiments. The material(s) forming electrodes 10 and
10' can be porous, such that they have a high surface area for
contacting first electrolyte solution 30 and second electrolyte
solution 40, the active materials of which are capable of cycling
between an oxidized state and a reduced state during operation of
flow battery 1. For example, one or both of electrodes 10 and 10'
can be formed from a porous carbon cloth or a carbon foam in
particular embodiments.
[0026] Pump 60 affects transport of first electrolyte solution 30
containing a first active material from tank 50 to the
electrochemical cell. The flow battery also suitably includes
second tank 50'' that holds second electrolyte solution 40
containing a second active material. The second active material in
second electrolyte solution 40 can be the same material as the
first active material in first electrolyte solution 30, or it can
be different. Second pump 60' can affect transport of second
electrolyte solution 40 to the electrochemical cell. Pumps (not
shown in FIG. 1) can also be used to affect transport of the first
and second electrolyte solutions 30 and 40 from the electrochemical
cell back to tanks 50 and 50'. Other methods of affecting fluid
transport, such as siphons, for example, can also suitably
transport first and second electrolyte solutions 30 and 40 into and
out of the electrochemical cell. Also shown in FIG. 1 is power
source or load 70, which completes the circuit of the
electrochemical cell and allows a user to collect or store
electricity during its operation. Connection to the electrical grid
for charging or discharging purposes can also occur at this
location.
[0027] It should be understood that FIG. 1 depicts a specific,
non-limiting embodiment of a flow battery. Accordingly, flow
batteries consistent with the spirit of the present disclosure can
differ in various aspects relative to the configuration of FIG. 1.
As one example, a flow battery can include one or more active
materials that are solids, gases, and/or gases dissolved in
liquids. Active materials can be stored in a tank, in a vessel open
to the atmosphere, or simply vented to the atmosphere.
[0028] As indicated above, multiple electrochemical cells can also
be combined with one another in an electrochemical cell stack in
order to increase the rate that energy can be stored and released
during operation. The amount of energy released is determined by
the overall amounts of active materials that are present. An
electrochemical cell stack utilizes bipolar plates between adjacent
electrochemical cells to establish electrical communication but not
fluid communication between the two cells across the bipolar plate.
Thus, bipolar plates contain the electrolyte solutions in an
appropriate half-cell within the individual electrochemical cells.
Bipolar plates are generally fabricated from electrically
conductive materials that are fluidically non-conductive on the
whole. Suitable materials can include carbon, graphite, metal, or a
combination thereof. Bipolar plates can also be fabricated from
non-conducting polymers having a conductive material dispersed
therein, such as carbon particles or fibers, metal particles or
fibers, graphene, and/or carbon nanotubes. Although bipolar plates
can be fabricated from the same types of conductive materials as
can the electrodes of an electrochemical cell, they can lack the
continuous porosity permitting an electrolyte solution to flow
completely through the latter. It should be recognized that bipolar
plates are not necessarily entirely non-porous entities, however.
Bipolar plates can have innate or designed flow channels, for
example, that provide a greater surface area for allowing an
electrolyte solution to contact the bipolar plate. Suitable flow
channel configurations can include, for example, interdigitated
flow channels. In some embodiments, the flow channels can be used
to promote delivery of an electrolyte solution to an electrode
within the electrochemical cell.
[0029] An electrolyte solution can be delivered to and withdrawn
from each electrochemical cell via an inlet manifold and an outlet
manifold (not shown in FIG. 1). In some embodiments, the inlet
manifold and the outlet manifold can provide and withdraw an
electrolyte solution via the bipolar plates separating adjacent
electrochemical cells. Separate inlet manifolds can provide each
electrolyte solution individually to the two half-cells of each
electrochemical cell. Likewise, separate outlet manifolds withdraw
the electrolyte solutions from the positive and negative
half-cells. In more particular embodiments, the inlet manifold and
the outlet manifold can be configured to supply and withdraw the
electrolyte solutions via opposing lateral faces of the bipolar
plates (e.g. by supplying and withdrawing the electrolyte solution
from opposing ends of the flow channels within the bipolar plate).
Thus, the electrolyte solutions circulate laterally through the
individual half-cells of the flow battery system.
[0030] As indicated above, the concentration of one or more active
materials or other components in an electrolyte solution of a flow
battery system can change in concentration during prolonged
operation of the flow battery system and possibly reach
out-of-range values. For example, solvent loss can occur when
venting the flow battery system to remove gaseous reaction
products, which can be formed during parasitic reactions. Solvent
migration between the two electrolyte solutions can also occur if
the solvent potentials are different in the two half-cells.
Out-of-range concentration values can result in inefficient
operation of the flow battery system or precipitation from the
electrolyte solution in some instances. Although out-of-range
concentration values can be addressed through manually introducing
solvent to the electrolyte solution or heating the electrolyte
solution to evaporate a portion of the solvent, both approaches can
be problematic, particularly when performed on an electrolyte
solution contained within a closed circulation loop of a flow
battery system. Evaporative approaches employing heating can
consume substantial energy, which can lower the overall operating
efficiency of the flow battery system when considered on the whole.
In addition, measuring concentrations to determine the amount of
concentration or dilution needing to take place can be difficult in
its own right.
[0031] The present inventors discovered that an electrochemical
balancing cell disposed in fluid communication with either the
positive or negative electrolyte solution of a flow battery system
can be used to maintain the electrolyte solution at a desired
concentration level, in addition to maintaining state of charge
balance. Specifically, the inventors discovered that a two-chamber
balancing cell can be used to either add or remove solvent from an
electrolyte solution in a flow battery system, thereby increasing
or decreasing the concentration of one or more components therein.
Advantageously, such use of an electrochemical balancing cell can
take place in concert with the balancing cell's conventional
function of maintaining state of charge balance, as discussed
hereinafter. Water can be migrated to maintain concentrations in
aqueous electrolyte solutions, and other solvents can be migrated
similarly in non-aqueous electrolyte solutions.
[0032] During operation of a flow battery system, maintaining the
electrolyte solutions in charge balance with one another is usually
desirable. A balanced state of charge usually occurs when the
active material in a first electrolyte solution is oxidized and the
active material in a second electrolyte solution is concurrently
reduced in productive reactions, thereby maintaining the two
electrolyte solutions in state of charge balance with one another.
The term "state of charge" (SOC) is a well understood
electrochemical energy storage term and refers to the relative
amounts of reduced and oxidized species at an electrode within a
given half-cell of an electrochemical system. As used herein, the
term "productive reactions" refer to electrochemical reactions of
flow battery active materials that contribute to the flow battery's
proper operation during charging and discharging cycles. If only
productive reactions occurred in a flow battery system, the
electrolyte solutions would continually remain in a charge balanced
state.
[0033] Undesirable parasitic reactions can also occur within one or
both half-cells of flow battery systems that can upset the desired
state of charge balance. As used herein, the term "parasitic
reaction" refers to any side electrochemical reaction that takes
place in conjunction with productive reactions. Parasitic reactions
can involve any component of an electrolyte solution that is not
the active material, particularly the solvent of the electrolyte
solution. Electrochemical reactions of an active material that
render the active material unable to undergo reversible oxidation
and reduction can also be considered parasitic in nature. Parasitic
reactions that commonly occur in aqueous electrolyte solutions are
reduction of water into hydrogen at the negative electrode and/or
oxidation of water into oxygen at the positive electrode.
Furthermore, parasitic reactions in aqueous electrolyte solutions
can change the electrolyte solution's pH, which can destabilize the
active material in some instances. Hydrogen evolution in a negative
electrolyte solution, for example, can raise the pH by consuming
protons and forming hydroxide ions.
[0034] Discharge arising from parasitic reactions can decrease the
operating efficiency and other performance parameters of flow
battery system. In the case of a parasitic reaction that occurs
preferentially in one half-cell over the other, an imbalance in
state of charge can result between the negative and positive
electrolyte solutions. Charge imbalance between the electrolyte
solutions of a flow battery system can lead to mass transport
limitations at one of the electrodes, thereby lowering the
round-trip operating efficiency. Since the charge imbalance can
grow with each completed charge and discharge cycle, increasingly
diminished performance of a flow battery system can result over
time due to parasitic reactions. Parasitic generation of hydrogen
at a negative electrode can further result in undercharging or
partial discharging of the negative electrolyte solution, which can
produce a state of charge imbalance.
[0035] Types of parasitic reactions that can occur in flow battery
systems containing aqueous electrolyte solutions include, for
example, generation of hydrogen and oxidation by oxygen. Hydrogen
generation in the negative electrolyte solution of flow batteries
can be especially problematic due to pH changes and the state of
charge imbalance accompanying this parasitic reaction. Parasitic
evolution of hydrogen in the negative half-cell of a flow battery
system can occur as shown in Reaction 1 below.
2H.sub.2O+2e.sup.-.fwdarw.H.sub.2+20H.sup.- (Reaction 1)
During ideal charging conditions, all current passing through the
flow battery system charges the active materials in the negative
and positive electrolyte solutions. When Reaction 1 occurs,
however, a fraction of the current promotes hydrogen evolution, not
charging of the active material in the negative electrolyte
solution. At the end of the charging cycle, the state of charge of
the negative electrolyte solution is lower than that of the
positive electrolyte solution, assuming no parasitic reactions
occurred in the positive electrolyte solution. The extent of the
state of charge imbalance can increase over successive charge and
discharge cycles.
[0036] Conventional approaches for rectifying a state of charge
imbalance between two electrolyte solutions can involve reducing
the active material in the negative electrolyte solution within one
half-cell of a two-chamber electrochemical balancing cell until the
two electrolyte solutions are brought back into charge balance with
one another. As used herein, the term "electrochemical balancing
cell" refers to a sub-system of a flow battery system in which the
state of charge of one electrolyte solution can be adjusted without
simultaneously changing the state of charge of the other
electrolyte solution. A typical electrochemical balancing cell
contains a separator disposed between a first half-cell and a
second half-cell, in which the electrolyte solution is contained or
circulated through one half-cell and a rebalancing aqueous fluid is
contained or circulated through the second half-cell. As such, a
two-chamber electrochemical balancing cell bears some similarity to
the individual electrochemical cells of a flow battery system, but
does not charge or discharge both active materials simultaneously,
given that only one active material is circulated through the
electrochemical balancing cell. Further description of two-chamber
electrochemical balancing cells that can be employed in the various
embodiments of the present disclosure can be found in U.S. Patent
Application Publication 2016/0233531, which is incorporated herein
by reference in its entirety. Three-chamber electrochemical
balancing cells can also be utilized in the present disclosure in
some instances.
[0037] Oxidation of water is performed using conventional balancing
cell approaches (i.e., two-chamber balancing cell approaches) in
the half-cell opposite that in which the electrolyte solution from
the flow battery system is contained or circulated. In one
balancing approach, water is oxidized to oxygen and protons in a
rebalancing aqueous fluid under the mediation of an
oxygen-generation catalyst, and the active material in the negative
electrolyte solution undergoes reduction in a corresponding
half-reaction within the other half-cell of the two-chamber
electrochemical balancing cell. Protons generated from the
oxidation of water can migrate across the membrane to offset the
increased negative charge resulting from reduction of the active
material in the negative electrolyte solution. Operating voltages
of up to approximately 3.5 V are used in this conventional approach
to a promote state of charge balancing. In more specific
embodiments, the operating voltage can range between about 2.4 V to
about 3.5 V.
[0038] In addition to the migration of protons across the membrane,
water can also cross the membrane under the influence of the
applied potential. Non-aqueous solvents can migrate similarly under
the applied potential. In addition to the applied potential, the
rate of water or other solvent migration can be impacted by the
activities (chemical potential) of the solvent upon either side of
the membrane in the electrochemical balancing cell. The present
inventors recognized that the migration of water or other solvent
into the electrolyte solution in the electrochemical balancing cell
could also be exploited to alter the concentrations of the active
material and other components in the electrolyte solution, thereby
offsetting concentration changes arising from loss of water or
other solvent during operation of the flow battery system. The
inventors discovered that the rate of water migration from the
balancing aqueous fluid to the electrolyte solution is a function
of the amount of current applied to the electrochemical balancing
cell. As such, the amount of applied current can be adjusted to
control the amount of water being introduced to the electrolyte
solution, thereby increasing its volume and decreasing the
concentrations of various components present in the electrolyte
solution. Further advantageously, such introduction of water to the
electrolyte solution can occur in concert with charge rebalancing,
provided the applied voltage is sufficiently high to promote the
catalytic production of oxygen at a high enough rate. If only a
concentration adjustment of the electrolyte solution is needed,
however, water migration at lower voltages can also be accomplished
without altering the state of charge.
[0039] The present inventors also surprisingly discovered that
water migration in the opposite direction within the
electrochemical balancing cell can also be realized, thereby
increasing the concentration of the active material or other
components in the electrolyte solution. Water migration from the
electrolyte solution of the flow battery system to the opposing
half-cell in the electrochemical balancing cell can take place by
operating the electrochemical balancing cell with one half-cell
empty or otherwise devoid of the balancing aqueous fluid. Although
it might seem counterintuitive to operate the electrochemical
balancing cell with only one half-cell filled with fluid (i.e., the
electrolyte solution), the inventors found that a current could
still be applied thereto without harming the cell components. In
particular, the membrane in the electrochemical balancing cell can
be configured to withstand the hydrostatic pressures present when
one half-cell is empty (e.g., through designed flow fields,
membrane hydration, small areas of unsupported membrane contact
with the electrolyte solution, and/or low operating pressures).
Once a sufficient amount of water has been removed from the
electrolyte solution, normal operation of the electrochemical
balancing cell can then resume. The cycles of water introduction
and removal can take place iteratively as needed during
operation.
[0040] Further advantageously, operation of the electrochemical
balancing cell can be automated in a feedback loop without actually
having to determine the concentration of the active material or any
other component within the electrolyte solution. Specifically,
provided that a known quantity of the electrolyte solution is
loaded in the flow battery system, the quantity of the electrolyte
solution can be measured through various types of sensors and
relayed to automated processing controls configured to operate the
electrochemical balancing cell in a particular manner. For example,
the weight or volume of the electrolyte solution can be monitored
at one or more locations in the flow battery system and relayed to
the automated processing controls to direct operation of the
electrochemical balancing cell. At this point, a processor within
the automated processing controls can direct operation of the
electrochemical balancing cell in a desired manner to either add or
remove water from the electrolyte solution, as discussed above. Of
course, in alternative embodiments, the quantity of the electrolyte
solution can also be measured or observed manually, and/or
regulation of the electrochemical balancing cell can take place
with operator intervention.
[0041] By practicing the disclosure herein, additional advantages
can also be realized. In conditions wherein one half-cell of the
electrochemical balancing cell remains empty, savings resulting
from water supply and pump downtime can be realized. In addition,
pump and/or water supply maintenance can also take place during
this time. A reduction in other subsystems can also be realized by
employing the disclosure herein. For example, water can be
alternatively supplied or removed by an additional reverse osmosis
cell, but this can significantly increase cost. Supplying one of
the fluids in the electrochemical balancing cell at a higher
pressure can also push solvent in a desired direction, but this can
result in energy and cost inefficiencies.
[0042] Finally, a second electrochemical balancing cell can also be
employed to independently alter the amount of water present in the
other electrolyte solution of the flow battery system. Although
only one electrochemical balancing cell is usually needed in
conventional approaches to maintain the two electrolyte solutions
in a balanced state of charge (i.e., one electrolyte solution can
be altered to match the state of charge of the other), it can be
advantageous to operate two electrochemical balancing cells in the
present disclosure to maintain independent concentration control of
both electrolyte solutions. Operation and control of the second
electrochemical balancing cell can take place in a manner similar
to that described above.
[0043] Accordingly, flow battery systems of the present disclosure
can contain a first half-cell containing a first aqueous
electrolyte solution, a second half-cell containing a second
aqueous electrolyte solution, a first electrochemical balancing
cell containing a membrane disposed between two half-cells, and a
source of a balancing aqueous fluid in fluid communication with the
electrochemical balancing cell. The first half-cell of the flow
battery system is in fluid communication with a first half-cell of
the electrochemical balancing cell. The flow battery system is
configured to introduce the balancing aqueous fluid to a second
half-cell of the electrochemical balancing cell when a quantity of
the first aqueous electrolyte solution falls below a lower
threshold and to withdraw the balancing aqueous fluid from the
second half-cell of the electrochemical balancing cell when the
volume of the first aqueous electrolyte solution exceeds an upper
threshold. The upper and lower thresholds are arbitrary and can
vary based on the intended application in which the flow battery is
employed.
[0044] The fluid communication between the electrochemical
balancing cell and first half-cell of the flow battery system can
be established at any location within the architecture of the flow
battery system. For example, FIG. 2 shows the illustrative flow
battery system of FIG. 1 further incorporating electrochemical
balancing cells in fluid communication with each electrolyte
solution via fluid connections off tanks 30 and 40. In some
embodiments, an electrochemical balancing cell is in fluid
communication with each electrolyte solution, and in other
embodiments, an electrochemical balancing cell is in fluid
communication with only one of the electrolyte solutions of the
flow battery system. Further description of FIG. 2 follows below.
Common reference characters will be used to denote elements
previously described in FIG. 1.
[0045] Referring to FIG. 2, electrochemical balancing cell 80 is in
fluid communication with tank 30, and electrochemical balancing
cell 80' is in fluid communication with tank 40. Flow battery
system 100 further includes reservoirs 90 and 90' in fluid
communication with electrochemical balancing cells 80 and 80',
respectively. Reservoirs 90 and 90' contain a balancing aqueous
fluid, such as water or an electrolyte solution. Non-aqueous
balancing fluids can alternately be used to adjust concentrations
in non-aqueous electrolyte solutions. Flow battery system 100 is
configured to circulate electrolyte solution from tanks 30 and 40
through one half-cell of electrochemical balancing cells 80 and
80'. Flow battery system 100 is also configured to circulate the
balancing aqueous fluid (when needed) through the other half-cell
of electrochemical balancing cells 80 and 80' from reservoirs 90
and 90'. Again, it is to be emphasized that the manner in which
electrochemical balancing cells 80 and 80' are placed in fluid
communication with the electrolyte solutions containing the active
materials of the flow battery system can vary with system design.
As such, the configuration of FIG. 2 should be considered
non-limiting.
[0046] As mentioned above, various processing controls can be
present in order to monitor and regulate the amount of one or more
of the aqueous electrolyte solutions that are present in the flow
battery systems of the present disclosure. In more specific
embodiments, the flow battery systems can include a processor that
is responsive to the amount of the first and/or second aqueous
electrolyte solution and is configured to initiate introduction or
withdrawal of the first and/or second aqueous electrolyte solution
to or from the second half-cell of the electrochemical balancing
cell(s).
[0047] Suitable processing controls can incorporate various blocks,
modules, elements, components, methods and algorithms, which can be
implemented through using computer hardware, software and
combinations thereof. To illustrate this interchangeability of
hardware and software, various illustrative blocks, modules,
elements, components, methods and algorithms have been described
generally in terms of their functionality. Whether such
functionality is implemented as hardware or software will depend
upon the particular application and any imposed design constraints.
For at least this reason, it is to be recognized that one of
ordinary skill in the art can implement the described functionality
in a variety of ways for a particular application. Further, various
components and blocks can be arranged in a different order or
partitioned differently, for example, without departing from the
spirit and scope of the embodiments expressly described.
[0048] Computer hardware used to implement the various illustrative
blocks, modules, elements, components, methods and algorithms
described herein can include a processor configured to execute one
or more sequences of instructions, programming or code stored on a
readable medium. The processor can be, for example, a general
purpose microprocessor, a microcontroller, a digital signal
processor, an application specific integrated circuit, a field
programmable gate array, a programmable logic device, a controller,
a state machine, a gated logic, discrete hardware components, an
artificial neural network or any like suitable entity that can
perform calculations or other manipulations of data. In some
embodiments, computer hardware can further include elements such
as, for example, a memory [e.g., random access memory (RAM), flash
memory, read only memory (ROM), programmable read only memory
(PROM), erasable PROM], registers, hard disks, removable disks,
CD-ROMs, DVDs, or any other like suitable storage device.
[0049] Executable sequences described herein can be implemented
with one or more sequences of code contained in a memory. In some
embodiments, such code can be read into the memory from another
machine-readable medium. Execution of the sequences of instructions
contained in the memory can cause a processor to perform the
process steps described herein. One or more processors in a
multi-processing arrangement can also be employed to execute
instruction sequences in the memory. In addition, hard-wired
circuitry can be used in place of or in combination with software
instructions to implement various embodiments described herein.
Thus, the present embodiments are not limited to any specific
combination of hardware and software.
[0050] As used herein, a machine-readable medium refers to any
medium that directly or indirectly provides instructions to a
processor for execution. A machine-readable medium can take on many
forms including, for example, non-volatile media, volatile media,
and transmission media. Non-volatile media can include, for
example, optical and magnetic disks. Volatile media can include,
for example, dynamic memory. Transmission media can include, for
example, coaxial cables, wire, fiber optics, and wires that form a
bus. Common forms of machine-readable media can include, for
example, floppy disks, flexible disks, hard disks, magnetic tapes,
other like magnetic media, CD-ROMs, DVDs, other like optical media,
punch cards, paper tapes and like physical media with patterned
holes, RAM, ROM, PROM, EPROM and flash EPROM.
[0051] In some embodiments, the flow battery systems of the present
disclosure can include a detector configured to determine the
quantity of the first and/or second aqueous electrolyte solution
that is present in the flow battery system. Determination of the
quantity of the first and/or second aqueous electrolyte solution
can take place by measurement of the mass of the first and/or
second aqueous electrolyte solution or by measuring the volume of
the first and/or second aqueous electrolyte solution. For example,
volume determination can take place in the tank in which the first
and/or second aqueous electrolyte solution is stored, such as
through using a float or other suitable apparatus for determining a
liquid level in the tank. Other suitable detectors can include, for
example, sonic level sensors, liquid column or differential
pressure sensors, vibrating fork sensors or switches, capacity
liquid level sensors, and the like. Optical sensors capable of
measuring a concentration of one or more components in the
electrolyte solution can also be used. Density measurement by
determining the head pressure on the electrolyte solution can also
be employed. Mass determination can be performed, for example, by
weighing the tanks in which the first and/or second electrolyte
solution is housed. The processor or processing controls described
above can obtain an input from the detector and then regulate the
operation of the electrochemical balancing cell(s) based upon the
output of the detector. More particularly, in some embodiments, the
processor or processing controls can be responsive to the volume(s)
of the first and/or second aqueous electrolyte solution to initiate
introduction or withdrawal of the first and/or second aqueous
electrolyte solution to or from the second half-cell of the
electrochemical balancing cell(s).
[0052] In various embodiments, an oxygen-generation catalyst can be
present in the second half-cell of the electrochemical balancing
cell(s). As used herein, the term "oxygen-generation catalyst"
refers to a catalyst that is capable of converting water or
hydroxide ions into oxygen under an applied potential. Some
oxygen-generation catalysts can function under neutral or acidic
conditions and affect conversion of water into oxygen and protons.
Iridium oxide catalysts and iridium-ruthenium oxide catalysts or
other noble metal catalysts can be suitably used in this regard.
Other oxygen-generation catalysts can function under alkaline
conditions and affect conversion of hydroxide ions into oxygen and
water. Suitable oxygen-generation catalysts for oxidizing hydroxide
ions to oxygen under alkaline conditions include, for example,
nickel or nickel-based catalysts. These types of oxygen-generation
catalysts can be advantageous due to their lower costs compared to
iridium-based oxygen-generation catalysts. Carbon-based catalysts
can also be used in some instances.
[0053] In view of the foregoing, the present disclosure also
describes methods for adding or removing solvent from an
electrolyte solution in a flow battery system in which at least one
electrochemical balancing cell is present. More specifically, the
present disclosure provides methods for increasing or decreasing
the volume of an aqueous electrolyte solution in a flow battery
system by using an electrochemical balancing cell. In some
embodiments, independent regulation of the volume of each
electrolyte solution in a flow battery system can take place by
employing a first electrochemical balancing cell in fluid
communication with the first aqueous electrolyte solution and a
second electrochemical balancing in fluid communication with the
second aqueous electrolyte solution.
[0054] In some embodiments, methods of the present disclosure can
include providing a first electrochemical balancing cell containing
a membrane disposed between two half-cells, establishing fluid
communication between a first aqueous electrolyte solution of a
flow battery system and a first half-cell of the first
electrochemical balancing cell, and applying a current to the first
electrochemical balancing cell. Applying the current causes water
to migrate across the membrane, either to or from the first aqueous
electrolyte solution. Migration of water from the first aqueous
electrolyte solution increases the concentration of the active
material and other components in the first aqueous electrolyte
solution through decreasing the volume. As discussed above, water
removal from the first aqueous electrolyte solution can be achieved
by operating the first electrochemical balancing cell with the
second half-cell of the first electrochemical balancing cell empty.
In contrast, migration of water to the first aqueous electrolyte
solution decreases the concentration of the active material and
other components in the first aqueous electrolyte solution through
increasing the volume. Water addition to the first aqueous
electrolyte solution can be accomplished by applying a current to
the first electrochemical balancing cell with a balancing aqueous
fluid present in the second half-cell of the first electrochemical
balancing cell. The rate of ingress or egress of water through the
membrane of the first electrochemical balancing cell into or out of
the first aqueous electrolyte solution can be adjusted through
altering the amount of current being applied to the first
electrochemical balancing cell.
[0055] In some or other various embodiments, methods of the present
disclosure can include providing a first electrochemical balancing
cell containing a membrane disposed between two half-cells,
establishing fluid communication between a first aqueous
electrolyte solution of a flow battery system and a first half-cell
of the first electrochemical balancing cell, determining a quantity
of the first aqueous electrolyte solution in the flow battery
system, applying a current to the first electrochemical balancing
cell, and either introducing a balancing aqueous fluid to a second
half-cell of the first electrochemical balancing cell or emptying
the second half-cell of the balancing aqueous fluid in response to
the quantity of the first aqueous electrolyte solution that is
determined. Applying the current causes water to migrate across the
membrane into the first aqueous electrolyte solution when the
balancing aqueous fluid is present in the second half-cell of the
first electrochemical balancing cell, and applying the current
causes water to migrate across the membrane into the second
half-cell when the balancing aqueous fluid is absent from the first
electrochemical balancing cell, such as when the second half-cell
of the first electrochemical balancing cell is empty. A rate of
water migration is a function of current applied to the
electrochemical balancing cell. Other factors such as the
electrolyte pressure, the balancing aqueous fluid pressure, and/or
the salt content of the electrolyte solution or the balancing
aqueous fluid can also impact migration rates.
[0056] In some embodiments, the second half-cell of the first
electrochemical balancing cell contains a balancing aqueous fluid,
in which case applying the current to the first electrochemical
balancing cell causes water to migrate from the second half-cell of
the first electrochemical balancing cell into the first aqueous
electrolyte solution in the first half-cell of the first
electrochemical balancing cell. As such, operating the first
electrochemical balancing cell in this manner decreases an active
material concentration in the first aqueous electrolyte solution,
as well as decreasing the concentration of other components in the
first aqueous electrolyte solution.
[0057] In more specific embodiments, the second half-cell of the
first electrochemical balancing cell can contain an
oxygen-generation catalyst. As such, in some embodiments, protons
generated in the second half-cell can also migrate to the first
aqueous electrolyte solution when a balancing aqueous fluid is
present in the first electrochemical balancing cell. When the
second half-cell of the first electrochemical balancing cell
contains an oxygen-generation catalyst, adjustment of pH of the
first aqueous electrolyte solution can also occur in conjunction
with increasing the volume of the first aqueous electrolyte
solution circulating through the first half-cell of the first
electrochemical balancing cell. The oxidation of water in the
balancing aqueous fluid represents the matching half-reaction of
the reduction of the active material in the first aqueous
electrolyte solution, thereby allowing state of charge adjustment
to take place. As such, adjustment of the state of charge and pH of
the first aqueous electrolyte solution can take place in
conjunction with increasing the volume of the first aqueous
electrolyte solution (decreasing the active material
concentration), in some embodiments herein. In other embodiments,
however, water migration across the membrane can take place at
lower current values at which reduction of the active material and
oxidation of water does not occur. Further discussion in this
regard follows hereinbelow.
[0058] In some embodiments, a balancing aqueous fluid that can be
present in the first electrochemical balancing cell is an aqueous
electrolyte solution. As used herein, the term "aqueous" refers to
the condition of water being the predominant component of a mixture
or solution. As used herein, the term "aqueous electrolyte
solution" refers to a homogeneous liquid phase containing water as
a predominant solvent in which one or more mobile ions and/or
active materials are present. Aqueous electrolyte solutions of the
present disclosure encompass both solutions in water and water
solutions containing a water-miscible organic solvent as a minority
component. Aqueous electrolyte solutions employed in the second
half-cell of the electrochemical balancing cell lack the active
materials present in the aqueous electrolyte solutions circulated
through the flow battery system for generating electricity. In
other embodiments of the present disclosure, the balancing aqueous
fluid can comprise water or consist essentially of water in which
mobile ions are not present. In still more specific embodiments,
the balancing aqueous fluid can consist of water alone. Optionally,
a water-miscible organic solvent can be present in combination with
water in embodiments in which mobile ions are not present.
[0059] Illustrative water-miscible organic solvents that can be
present in aqueous electrolyte solutions include, for example,
alcohols and glycols, optionally in the presence of one or more
surfactants or other components discussed below. In more specific
embodiments, an aqueous electrolyte solution can contain at least
about 98% water by weight. In other more specific embodiments, an
aqueous electrolyte solution can contain at least about 55% water
by weight, or at least about 60% water by weight, or at least about
65% water by weight, or at least about 70% water by weight, or at
least about 75% water by weight, or at least about 80% water by
weight, or at least about 85% water by weight, or at least about
90% water by weight, or at least about 95% water by weight. In some
embodiments, an aqueous electrolyte solution can be free of
water-miscible organic solvents and consist of water alone as a
solvent.
[0060] In further embodiments, an aqueous electrolyte solution can
include a viscosity modifier, a wetting agent, or any combination
thereof. Suitable viscosity modifiers can include, for example,
corn starch, corn syrup, gelatin, glycerol, guar gum, pectin, and
the like. Other suitable examples will be familiar to one having
ordinary skill in the art. Suitable wetting agents can include, for
example, various non-ionic surfactants and/or detergents. In some
or other embodiments, an aqueous electrolyte solution can further
include a glycol or a polyol. Suitable glycols can include, for
example, ethylene glycol, diethylene glycol, and polyethylene
glycol. Suitable polyols can include, for example, glycerol,
mannitol, sorbitol, pentaerythritol, and
tris(hydroxymethyl)aminomethane. Inclusion of any of these
components in an aqueous electrolyte solution can help promote
dissolution of a coordination complex or similar active material
and/or reduce viscosity of the aqueous electrolyte solution for
conveyance through a flow battery or electrochemical balancing
cell, for example.
[0061] In addition to a solvent, aqueous electrolyte solutions can
also include one or more mobile ions (i.e., an extraneous
electrolyte) in some embodiments. In some embodiments, suitable
mobile ions can include proton, hydronium, or hydroxide. In other
various embodiments, mobile ions other than proton, hydronium, or
hydroxide can be present, either alone or in combination with
proton, hydronium or hydroxide. Such alternative mobile ions can
include, for example, alkali metal or alkaline earth metal cations
(e.g., Li.sup.+, Na.sup.+, K.sup.+, Mg.sup.2+, Ca.sup.2+ and
Sr.sup.2+) and halides (e.g., F.sup.-, Cl.sup.-, or Br.sup.-).
Other suitable mobile ions can include, for example, ammonium and
tetraalkylammonium ions, chalcogenides, phosphate, hydrogen
phosphate, phosphonate, nitrate, sulfate, nitrite, sulfite,
perchlorate, tetrafluoroborate, hexafluorophosphate, and any
combination thereof. In some embodiments, less than about 50% of
the mobile ions can constitute protons, hydronium, or hydroxide. In
other various embodiments, less than about 40%, less than about
30%, less than about 20%, less than about 10%, less than about 5%,
or less than about 2% of the mobile ions can constitute protons,
hydronium, or hydroxide.
[0062] Upon introducing water into the first aqueous electrolyte
solution, as discussed above, it can become desirable in some cases
to subsequently remove a portion of the water from the first
aqueous electrolyte solution, such as if excess water is introduced
to the first aqueous electrolyte solution or if the operating
conditions of the flow battery system otherwise dictate a need to
decrease the volume of the first aqueous electrolyte solution. In
some embodiments, a signal to decrease the volume of the first
aqueous electrolyte solution can occur via a feedback loop through
appropriate processing controls, and in other embodiments, the
signal to decrease the volume can take place through manual
operator intervention. In either case, the methods of the present
disclosure can facilitate a rapid alteration between supplying the
balancing aqueous fluid to the second half-cell of the first
electrochemical balancing cell or removing the balancing aqueous
fluid to promote introduction or removal of water to or from the
first aqueous electrolyte solution, as appropriate. More
specifically, in some embodiments, methods of the present
disclosure can include removing the balancing aqueous fluid from
the second half-cell of the first electrochemical balancing cell,
and after removing the balancing aqueous fluid, applying the
current to the first electrochemical balancing cell while the
second half-cell is empty. In such embodiments, applying the
current causes water to migrate from the first half-cell into the
empty second half-cell, thereby increasing an active material
concentration or a concentration of another component in the first
aqueous electrolyte solution.
[0063] In other embodiments, the first electrochemical balancing
cell can be operated such that water is removed from the first
aqueous electrolyte solution and then subsequently re-introduced to
the first aqueous electrolyte solution, if necessary.
Re-introduction of water to the first aqueous electrolyte solution
can take place, for example, if excess water has been removed
therefrom and/or if operating conditions otherwise dictate a
subsequent increase in volume of the first aqueous electrolyte
solution. Accordingly, some methods of the present disclosure can
include those in which the second half-cell of the first
electrochemical balancing cell is left empty, and applying the
current to the first electrochemical balancing cell causes water to
migrate from the first half-cell into the second half-cell, thereby
increasing an active material concentration or a concentration of
another component in the first aqueous electrolyte solution.
Subsequently, the methods can further include introducing a
balancing aqueous fluid into the second half-cell of the first
electrochemical balancing cell, and after introducing the balancing
aqueous fluid, applying the current to the first electrochemical
balancing cell. As described above, applying the current to the
first electrochemical balancing cell when the balancing aqueous
fluid is present causes water to migrate from the second half-cell
of the first electrochemical balancing cell into the first
half-cell of the first electrochemical balancing cell, thereby
decreasing an active material concentration or a concentration of
another component in the first aqueous electrolyte solution. Again,
the second half-cell of the first electrochemical balancing cell
can contain an oxygen-generation catalyst, regardless of whether
the rebalancing aqueous fluid is present or not.
[0064] In more specific embodiments, the first aqueous electrolyte
solution can be circulated through a negative half-cell of the flow
battery system. The negative half-cell of the flow battery system
can include a negative electrode, and the corresponding positive
half-cell of the flow battery system can include a positive
electrode. As used herein, the terms "negative electrode" and
"positive electrode" are electrodes defined with respect to one
another, such that the negative electrode operates or is designed
or intended to operate at a potential more negative than the
positive electrode (and vice versa), independent of the actual
potentials at which they operate, in both charging and discharging
cycles. The negative electrode may or may not actually operate or
be designed or intended to operate at a negative potential relative
to a reversible hydrogen electrode. Use of an electrochemical
balancing cell to modify at least the first aqueous electrolyte
solution in contact with the negative electrode, as provided above,
can be particularly desirable, given the greater propensity for
this electrolyte solution to undergo parasitic reactions, as
discussed herein. Although a second electrochemical balancing cell
is not necessarily needed in conventional flow battery systems in
which only state of charge rebalancing needs to take place, it can
be desirable to include one in some embodiments herein, so that the
volumes of the first aqueous electrolyte solution and the second
aqueous electrolyte solution can be independently regulated with
respect to one another.
[0065] The membrane present in the first electrochemical balancing
cell is not considered to be particularly limited. Suitable
membranes can include both cation-exchange membranes and
anion-exchange membranes. Negatively charged cation-exchange
membranes can be particularly suitable membranes for use in
contacting an aqueous electrolyte solution containing a negatively
charged active material. Charge matching between the membrane and
the first aqueous electrolyte solution can slow or preclude
crossover of the active material into the second half-cell of the
first electrochemical balancing cell, thereby preserving the active
material in the first aqueous electrolyte solution. Similar charge
matching can be employed in the membrane dividing the two
half-cells of the flow battery system in which the first and second
aqueous electrolyte solutions are separately circulating, as
discussed herein.
[0066] Suitable cation-exchange membranes in the first
electrochemical balancing cell or between the half-cells of the
flow battery system are not considered to be particularly limited.
Particularly suitable cation-exchange membranes can frequently bear
sulfonic acid groups due to their high degree of disassociation
into sulfonate anions. Accordingly, in some embodiments, the
cation-exchange membrane can include a sulfonated polymer, such as
a sulfonated, perfluorinated polymer. NAFION (DuPont) is
representative example of such a cation-exchange membrane. In other
embodiments, the cation-exchange membrane can be a sulfonated
hydrocarbon, such as a sulfonated polyetheretherketone or a
sulfonated polysulfone.
[0067] In other embodiments, anion-exchange membranes can be
included in the first electrochemical balancing cell or between the
two half-cells of the flow battery system. Suitable anion-exchange
membranes can include those bearing, for example, quaternary
ammonium functional groups or phosphonium groups.
[0068] In still other embodiments, bipolar membranes can be present
in the first electrochemical balancing cell or between the two
half-cells of the flow battery system. As used herein, the term
"bipolar membrane" is a membrane structure including both a
cation-exchange membrane and an anion-exchange membrane. Any
combination of cation-exchange and anion-exchange membranes can be
used.
[0069] In illustrative embodiments, the electrochemical balancing
cell can be operated at a current density of up to about 100
mA/cm.sup.2. When an oxygen-generation catalyst is present, oxygen
can be generated within this range. At the upper end of this range,
the rate of oxygen generation can be significant if an
oxygen-generation catalyst is present.
[0070] In certain embodiments, at least one of the first aqueous
electrolyte solution and the second aqueous electrolyte solution
can contain an active material that is a coordination complex. In
some embodiments, both the first aqueous electrolyte solution and
the second aqueous electrolyte solution can contain coordination
complexes, where the coordination complexes differ from one
another. Additional disclosure on illustrative coordination
complexes follows hereinafter.
[0071] In some embodiments, flow batteries of the present
disclosure can include an active material that is a coordination
complex in one or more of the aqueous electrolyte solutions. Due to
their variable oxidation states, transition metals can be highly
desirable for use within the active materials of a flow battery
system. Lanthanide metals can be used similarly in alternative
embodiments. Cycling between the accessible oxidation states can
result in the conversion of chemical energy into electrical energy.
Suitable metals can include, for example, Al, Ca, Co, Cr, Cu, Fe,
Hf, Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn Ti, Zn, Zr, V, W and U.
Especially desirable transition metals for inclusion in a flow
battery system include, for example, Al, Cr, Ti and Fe,
particularly in the form of a coordination complex. For purposes of
the present disclosure, Al is to be considered a transition metal.
In some embodiments, coordination complexes within a flow battery
can include at least one catecholate or substituted catecholate
ligand.
[0072] Other ligands that can be present in coordination complexes,
alone or in combination with one or more catecholate or substituted
catecholate ligands, include, for example, ascorbate, citrate,
glycolate, a polyol, gluconate, hydroxyalkanoate, acetate, formate,
benzoate, malate, maleate, phthalate, sarcosinate, salicylate,
oxalate, urea, polyamine, aminophenolate, acetylacetonate, and
lactate. Where chemically feasible, it is to be recognized that
such ligands can be optionally substituted with at least one group
selected from among C.sub.1-6 alkoxy, C.sub.1-6 alkyl, C.sub.1-6
alkenyl, C.sub.1-6 alkynyl, 5- or 6-membered aryl or heteroaryl
groups, a boronic acid or a derivative thereof, a carboxylic acid
or a derivative thereof, cyano, halide, hydroxyl, nitro, sulfonate,
a sulfonic acid or a derivative thereof, a phosphonate, a
phosphonic acid or a derivative thereof, or a glycol, such as
polyethylene glycol. Alkanoate includes any of the alpha, beta, and
gamma forms of these ligands. Polyamines include, but are not
limited to, ethylenediamine, ethylenediamine tetraacetic acid
(edta), and diethylenetriamine pentaacetic acid (dtpa).
[0073] Other examples of ligands can be present include
monodentate, bidentate, and/or tridentate ligands. Examples of
monodentate ligands that can be present in a coordination complex
include, for example, carbonyl or carbon monoxide, nitride, oxo,
hydroxo, water, sulfide, thiols, pyridine, pyrazine, and the like.
Examples of bidentate ligands that can be present in a coordination
complex include, for example, bipyridine, bipyrazine,
ethylenediamine, diols (including ethylene glycol), and the like.
Examples of tridentate ligands that can be present a coordination
complex include, for example, terpyridine, diethylenetriamine,
triazacyclononane, tris(hydroxymethyl)aminomethane, and the
like.
[0074] In some embodiments, one or more of the active materials can
be coordination complexes having a formula of
D.sub.gM(L.sub.1)(L.sub.2)(L.sub.3),
wherein D is an alkali metal ion, an ammonium ion, a
tetraalkylammonium ion, a phosphonium ion or any combination
thereof, g is an integer or non-integer value ranging between 1 and
6, M is a transition metal or lanthanide metal, and L.sub.1-L.sub.3
are bidentate ligands, such as those defined hereinabove. The value
of g can depend upon whether L.sub.1-L.sub.3 bear an ionic charge.
In some embodiments, at least one of L.sub.1-L.sub.3 can be a
catecholate ligand or substituted catecholate ligand, and in other
embodiments, each of L.sub.1-L.sub.3 is a catecholate ligand or a
substituted catecholate ligand. In some or other embodiments, M is
Ti. In embodiments in which M is Ti and L.sub.1-L.sub.3 are
uncharged catecholate ligands, g has a value of 2 to provide charge
balance against titanium (IV).
[0075] Flow battery systems of the present disclosure can
incorporate flow batteries that are capable of providing sustained
charge or discharge cycles of several hour durations. As such, they
can be used to smooth energy supply/demand profiles and provide a
mechanism for stabilizing intermittent power generation assets
(e.g., from renewable energy sources such as solar and wind
energy). It should be appreciated, then, that various embodiments
of the present disclosure include energy storage applications where
such long charge or discharge durations are desirable. For example,
in non-limiting examples, the flow batteries of the present
disclosure can be connected to an electrical grid to allow
renewables integration, peak load shifting, grid firming, baseload
power generation and consumption, energy arbitrage, transmission
and distribution asset deferral, weak grid support, frequency
regulation, or any combination thereof. When not connected to an
electrical grid, the flow batteries of the present disclosure can
be used as power sources for remote camps, forward operating bases,
off-grid telecommunications, remote sensors, the like, and any
combination thereof. Further, while the disclosure herein is
generally directed to flow batteries, it is to be appreciated that
other electrochemical energy storage media can incorporate the
electrolyte solutions and coordination complexes described herein,
including those utilizing stationary electrolyte solutions.
[0076] In some embodiments, flow batteries can include: a first
chamber containing a negative electrode contacting a first aqueous
electrolyte solution; a second chamber containing a positive
electrode contacting a second aqueous electrolyte solution, and a
separator/membrane disposed between the first and second aqueous
electrolyte solutions. The chambers provide separate reservoirs
within the flow battery, through which the first and/or second
aqueous electrolyte solutions circulate so as to contact the
respective electrodes. Each chamber and its associated electrode
and electrolyte solution define a corresponding half-cell. The
separator provides several functions which include, for example,
(1) serving as a barrier to mixing of the first and second aqueous
electrolyte solutions, (2) electrically insulating to reduce or
prevent short circuits between the positive and negative
electrodes, and (3) facilitating ion transport between the positive
and negative electrolyte chambers, thereby balancing electron
transport during charge and discharge cycles. The negative and
positive electrodes provide a surface where electrochemical
reactions can take place during charge and discharge cycles. During
a charge or discharge cycle, the electrolyte solutions can be
transported from separate storage tanks through the corresponding
chambers, as shown in FIGS. 1 and 3. In a charging cycle,
electrical power can be applied to the cell such that the active
material contained in the second aqueous electrolyte solution
undergoes a one or more electron oxidation and the active material
in the first aqueous electrolyte solution undergoes a one or more
electron reduction, or vice versa. Similarly, in a discharge cycle
the second active material is reduced and the first active material
is oxidized to generate electrical power, or vice versa.
[0077] The separator can be a porous membrane in some embodiments
and/or an ionomer membrane in other various embodiments. In some
embodiments, the separator can be formed from an ionically
conductive polymer.
[0078] Polymer membranes can be anion- or cation-conducting
electrolytes. Where described as an "ionomer," the term refers to
polymer membrane containing both electrically neutral repeating
units and ionized repeating units, where the ionized repeating
units are pendant and covalently bonded to the polymer backbone. In
general, the fraction of ionized units can range from about 1 mole
percent to about 90 mole percent. For example, in some embodiments,
the content of ionized units is less than about 15 mole percent;
and in other embodiments, the ionic content is higher, such as
greater than about 80 mole percent. In still other embodiments, the
ionic content is defined by an intermediate range, for example, in
a range of about 15 to about 80 mole percent. Ionized repeating
units in an ionomer can include anionic functional groups such as
sulfonate, carboxylate, and the like. These functional groups can
be charge balanced by, mono-, di-, or higher-valent cations, such
as alkali or alkaline earth metals. Ionomers can also include
polymer compositions containing attached or embedded quaternary
ammonium, sulfonium, phosphazenium, and guanidinium residues or
salts. Suitable examples will be familiar to one having ordinary
skill in the art.
[0079] In some embodiments, polymers useful as a separator can
include highly fluorinated or perfluorinated polymer backbones.
Certain polymers useful in the present disclosure can include
copolymers of tetrafluoroethylene and one or more fluorinated,
acid-functional co-monomers, which are commercially available as
NAFION.TM. perfluorinated polymer electrolytes from DuPont. Other
useful perfluorinated polymers can include copolymers of
tetrafluoroethylene and
FSO.sub.2--CF.sub.2CF.sub.2CF.sub.2CF.sub.2--O--CF.dbd.CF.sub.2,
FLEMION.TM. and SELEMION.TM..
[0080] Additionally, substantially non-fluorinated membranes that
are modified with sulfonic acid groups (or cation exchanged
sulfonate groups) can also be used. Such membranes can include
those with substantially aromatic backbones such as, for example,
polystyrene, polyphenylene, biphenyl sulfone (BPSH), or
thermoplastics such as polyetherketones and poly ethersulfones.
[0081] Battery-separator style porous membranes, can also be used
as the separator. Because they contain no inherent ionic conduction
capabilities, such membranes are typically impregnated with
additives in order to function. These membranes typically contain a
mixture of a polymer and inorganic filler, and open porosity.
Suitable polymers can include, for example, high density
polyethylene, polypropylene, polyvinyl idene difluoride (PVDF), or
polytetrafluoroethylene (PTFE). Suitable inorganic fillers can
include silicon carbide matrix material, titanium dioxide, silicon
dioxide, zinc phosphide, and ceria.
[0082] Separators can also be formed from polyesters,
polyetherketones, poly(vinyl chloride), vinyl polymers, and
substituted vinyl polymers. These can be used alone or in
combination with any previously described polymer.
[0083] Porous separators are non-conductive membranes which allow
charge transfer between two electrodes via open channels filled
with electrolyte. The permeability increases the probability of
active materials passing through the separator from one electrode
to another and causing cross-contamination and/or reduction in cell
energy efficiency. The degree of this cross-contamination can
depend on, among other features, the size (the effective diameter
and channel length), and character (hydrophobicity/hydrophilicity)
of the pores, the nature of the electrolyte, and the degree of
wetting between the pores and the electrolyte.
[0084] The pore size distribution of a porous separator is
generally sufficient to substantially prevent the crossover of
active materials between the two electrolyte solutions. Suitable
porous membranes can have an average pore size distribution of
between about 0.001 nm and 20 micrometers, more typically between
about 0.001 nm and 100 nm. The size distribution of the pores in
the porous membrane can be substantial. In other words, a porous
membrane can contain a first plurality of pores with a very small
diameter (approximately less than 1 nm) and a second plurality of
pores with a very large diameter (approximately greater than 10
micrometers). The larger pore sizes can lead to a higher amount of
active material crossover. The ability for a porous membrane to
substantially prevent the crossover of active materials can depend
on the relative difference in size between the average pore size
and the active material. For example, when the active material is a
metal center in a coordination complex, the average diameter of the
coordination complex can be about 50% greater than the average pore
size of the porous membrane. On the other hand, if a porous
membrane has substantially uniform pore sizes, the average diameter
of the coordination complex can be about 20% larger than the
average pore size of the porous membrane. Likewise, the average
diameter of a coordination complex is increased when it is further
coordinated with at least one water molecule. The diameter of a
coordination complex of at least one water molecule is generally
considered to be the hydrodynamic diameter. In such embodiments,
the hydrodynamic diameter is generally at least about 35% greater
than the average pore size. When the average pore size is
substantially uniform, the hydrodynamic radius can be about 10%
greater than the average pore size.
[0085] In some embodiments, the separator can also include
reinforcement materials for greater stability. Suitable
reinforcement materials can include nylon, cotton, polyesters,
crystalline silica, crystalline titania, amorphous silica,
amorphous titania, rubber, asbestos, wood or any combination
thereof.
[0086] Separators within the flow batteries can have a membrane
thickness of less than about 500 micrometers, or less than about
300 micrometers, or less than about 250 micrometers, or less than
about 200 micrometers, or less than about 100 micrometers, or less
than about 75 micrometers, or less than about 50 micrometers, or
less than about 30 micrometers, or less than about 25 micrometers,
or less than about 20 micrometers, or less than about 15
micrometers, or less than about 10 micrometers. Suitable separators
can include those in which the flow battery is capable of operating
with a current efficiency of greater than about 85% with a current
density of 100 mA/cm.sup.2 when the separator has a thickness of
100 micrometers. In further embodiments, the flow battery is
capable of operating at a current efficiency of greater than 99.5%
when the separator has a thickness of less than about 50
micrometers, a current efficiency of greater than 99% when the
separator has a thickness of less than about 25 micrometers, and a
current efficiency of greater than 98% when the separator has a
thickness of less than about 10 micrometers. Accordingly, suitable
separators include those in which the flow battery is capable of
operating at a voltage efficiency of greater than 60% with a
current density of 100 mA/cm.sup.2. In further embodiments,
suitable separators can include those in which the flow battery is
capable of operating at a voltage efficiency of greater than 70%,
greater than 80% or even greater than 90%.
[0087] The crossover rate of the first and second active materials
through the separator can be less than about 1.times.10.sup.-5 mol
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-6 mol
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-7 mol
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-9 mol
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-11 mol
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-13 awl
cm.sup.-2 day.sup.-1, or less than about 1.times.10.sup.-45 mol
cm.sup.-2 day.sup.-1.
[0088] The flow batteries can also include an external electrical
circuit in electrical communication with the first and second
electrodes. The circuit can charge and discharge the flow battery
during operation. Reference to the sign of the net ionic charge of
the first, second, or both active materials relates to the sign of
the net ionic charge in both oxidized and reduced forms of the
redox active materials under the conditions of the operating flow
battery. Further exemplary embodiments of a flow battery provide
that (a) the first active material has an associated net positive
or negative charge and is capable of providing an oxidized or
reduced form over an electric potential in a range of the negative
operating potential of the system, such that the resulting oxidized
or reduced form of the first active material has the same charge
sign (positive or negative) as the first active material and the
ionomer membrane also has a net ionic charge of the same sign and
(h) the second active material has an associated net positive or
negative charge and is capable of providing an oxidized or reduced
form over an electric potential in a range of the positive
operating potential of the system, such that the resulting oxidized
or reduced form of the second active material has the same charge
sign (positive or negative sign) as the second active material and
the ionomer membrane also has a net ionic charge of the same sign;
or both (a) and (b). The matching charges of the first and/or
second active materials and the ionomer membrane can provide a high
selectivity. More specifically, charge matching can provide less
than about 3%, less than about 2%, less than about 1%, less than
about 0.5%, less than about 0.2%, or less than about 0.1% of the
molar flux of ions passing through the ionomer membrane as being
attributable to the first or second active material. The term
"molar flux of ions" will refer to the amount of ions passing
through the ionomer membrane, balancing the charge associated with
the flow of external electricity/electrons. That is, the flow
battery is capable of operating or operates with the substantial
exclusion of the active materials by the ionomer membrane, and such
exclusion can be promoted through charge matching.
[0089] Flow batteries incorporated within the present disclosure
can have one or more of the following operating characteristics:
(a) where, during the operation of the flow battery, the first or
second active materials constitute less than about 3% of the molar
flux of ions passing through the ionomer membrane; (b) where the
round trip current efficiency is greater than about 70%, greater
than about 80%, or greater than about 90%; (c) where the round trip
current efficiency is greater than about 90%; (d) where the sign of
the net ionic charge of the first, second, or both active materials
is the same in both oxidized and reduced forms of the active
materials and matches that of the ionomer membrane, (e) where the
ionomer membrane has a thickness of less than about 100 .mu.m, less
than about 75 .mu.m, less than about 50 .mu.m, or less than about
250 .mu.m; (f) where the flow battery is capable of operating at a
current density of greater than about 100 mA/cm.sup.2 with a round
trip voltage efficiency of greater than about 60%; and (g) where
the energy density of the electrolyte solutions is greater than
about 10 Wh/L, greater than about 20 Wh/L, or greater than about 30
Wh/L.
[0090] In some cases, a user may desire to provide higher charge or
discharge voltages than are available from a single electrochemical
cell. In such cases, several battery cells can be connected in
series such that the voltage of each cell is additive. This forms a
bipolar stack, also referred to as an electrochemical stack. A
bipolar plate can be employed to connect adjacent electrochemical
cells in a bipolar stack, which allows for electron transport to
take place but prevents fluid or gas transport between adjacent
cells. The positive electrode compartments and negative electrode
compartments of individual cells can be fluidically connected via
common positive and negative fluid manifolds in the bipolar stack.
In this way, individual cells can be stacked in series to yield a
voltage appropriate for DC applications or conversion to AC
applications.
[0091] In additional embodiments, the cells, bipolar stacks, or
batteries can be incorporated into larger energy storage systems,
suitably including piping and controls useful for operation of
these large units. Piping, control, and other equipment suitable
for such systems are known in the art, and can include, for
example, piping and pumps in fluid communication with the
respective chambers for moving electrolyte solutions into and out
of the respective chambers and storage tanks for holding charged
and discharged electrolytes. The cells, cell stacks, and batteries
can also include an operation management system. The operation
management system can be any suitable controller device, such as a
computer or microprocessor, and can contain logic circuitry that
sets operation of any of the various valves, pumps, circulation
loops, and the like.
[0092] In more specific embodiments, a flow battery system can
include a flow battery (including a cell or cell stack); storage
tanks and piping for containing and transporting the electrolyte
solutions; control hardware and software (which may include safety
systems); and a power conditioning unit. The flow battery cell
stack accomplishes the conversion of charging and discharging
cycles and determines the peak power. The storage tanks contain the
positive and negative active materials, such as the coordination
complexes disclosed herein, and the tank volume determines the
quantity of energy stored in the system. The control software,
hardware, and optional safety systems suitably include sensors,
mitigation equipment and other electronic/hardware controls and
safeguards to ensure safe, autonomous, and efficient operation of
the flow battery system. A power conditioning unit can be used at
the front end of the energy storage system to convert incoming and
outgoing power to a voltage and current that is optimal for the
energy storage system or the application. For the example of an
energy storage system connected to an electrical grid, in a
charging cycle the power conditioning unit can convert incoming ac
electricity into dc electricity at an appropriate voltage and
current for the cell stack. In a discharging cycle, the stack
produces DC electrical power and the power conditioning unit
converts it to AC electrical power at the appropriate voltage and
frequency for grid applications.
[0093] Where not otherwise defined hereinabove or understood by one
having ordinary skill in the art, the definitions in the following
paragraphs will be applicable to the present disclosure.
[0094] As used herein, the term "energy density" refers to the
amount of energy that can be stored, per unit volume, in the active
materials. Energy density refers to the theoretical energy density
of energy storage and can be calculated by Equation 1:
Energy density=(26.8 A-h/mol).times.OCV.times.[e.sup.-] (1)
where OCV is the open circuit potential at 50% state of charge,
(26.8 A-h/mol) is Faraday's constant, and [e.sup.-] is the
concentration of electrons stored in the active material at 99%
state of charge. In the case that the active materials largely are
an atomic or molecular species for both the positive and negative
electrolyte, [e.sup.-] can be calculated by Equation 2 as:
[e.sup.-]=[active materials].times.N/2 (2)
where [active materials] is the molar concentration of the active
material in either the negative or positive electrolyte, whichever
is lower, and N is the number of electrons transferred per molecule
of active material. The related term "charge density" refers to the
total amount of charge that each electrolyte contains. For a given
electrolyte, the charge density can be calculated by Equation 3
Charge density=(26.8 A-h/mol).times.[active material].times.N
(3)
where [active material] and N are as defined above.
[0095] As used herein, the term "current density" refers to the
total current passed in an electrochemical cell divided by the
geometric area of the electrodes of the cell and is commonly
reported in units of mA/cm.sup.2.
[0096] As used herein, the term "current efficiency" (I.sub.eff)
can be described as the ratio of the total charge produced upon
discharge of a cell to the total charge passed during charging. The
current efficiency can be a function of the state of charge of the
flow battery. In some non-limiting embodiments, the current
efficiency can be evaluated over a state of charge range of about
35% to about 60%.
[0097] As used herein, the term "voltage efficiency" can be
described as the ratio of the observed electrode potential, at a
given current density, to the half-cell potential for that
electrode (.times.100%). Voltage efficiencies can be described for
a battery charging step, a discharging step, or a "round trip
voltage efficiency." The round trip voltage efficiency
(V.sub.eff,RT) at a given current density can be calculated from
the cell voltage at discharge (V.sub.discharge) and the voltage at
charge (V.sub.charge) using equation 4:
V.sub.eff,RT=V.sub.discharge/V.sub.charge.times.100% (4)
[0098] Although the disclosure has been described with reference to
the disclosed embodiments, those skilled in the art will readily
appreciate that these are only illustrative of the disclosure. It
should be understood that various modifications can be made without
departing from the spirit of the disclosure. The disclosure can be
modified to incorporate any number of variations, alterations,
substitutions or equivalent arrangements not heretofore described,
but which are commensurate with the spirit and scope of the
disclosure. Additionally, while various embodiments of the
disclosure have been described, it is to be understood that aspects
of the disclosure may include only some of the described
embodiments. Accordingly, the disclosure is not to be seen as
limited by the foregoing description.
* * * * *